You can reply to knowledge base articles, but only to comment on the article itself. If you have questions about your own situation, please create a topic in Hosting Support for that. I moved your post, but please keep this in mind next time.
As for your question itself, you can either add your own domain as a separate website (addon domain) or link it to your existing website (parked domain). Both methods work, but the setup varies a bit.
If you add the domain as a separate website, you’ll need to move the files first: How to move a website to a different directory. After that, you can just configure a Redirect for you old domain. No need to fiddle with .htaccess files for that, you can just use the Redirects tool in the client area.
If you go with the parked domain approach, you don’t need to migrate files, but setting up the redirect from the old domain to the new one might be tricky. Maybe your website software can take care of it, otherwise you’ll need some custom .htaccess rules.
Regarding the DNS configuration, there is no difference between the options. You can use our nameservers, or use your own nameservers and configure the required records yourself. No matter which of the two options above you choose, you’ll have a domain that’s linked to your hosting account, the only thing that really changes is the directory it’s linked to.